Acoustic gating monitor for magnetic resonance imaging system

ABSTRACT

An MRI imaging system provides an audible feedback signal in the gantry room triggered by a physiological sensor on the patient. The feedback signal is a sound generated by one of the MRI gradient coils. The sensor output signal may be indicative of the patient&#39;s heartbeat, or other physiological event. The application of the sequence causes the coil to emit a sound that is associated with the sensor output signal.

FIELD OF THE INVENTION

[0001] This invention relates generally to magnetic resonance (MR)imaging techniques. In particular, the invention relates to MR imagingthat is triggered and/or synchronized with patient sensors that detectphysiological conditions, such as a heartbeat, blood pulse, orrespiration of the patient being imaged.

BACKGROUND OF THE INVENTION

[0002] Magnetic Resonance Imaging (MRI) is a widely accepted andcommercially available technique for obtaining digitized visual imagesrepresenting the internal structure of objects (such as the human body)having substantial populations of atomic nuclei that are susceptible tonuclear magnetic resonance (MR) phenomena. In MRI, nuclei in the body ofa patient to be imaged are polarized by imposing a strong main magneticfield (BO) on the nuclei. The nuclei are excited by a radio frequency(RF) signal at characteristic MR (Lamor) frequencies. By spatiallydistributing localized magnetic fields surrounding the body andanalyzing the resulting RF responses from the nuclei, a map or image ofthese nuclei responses as a function of their spatial location isgenerated and displayed. An image of the nuclei responses provides anoninvasive view of a patient's internal organs and of other tissues.

[0003] As shown in FIG. 1, an MR imaging system typically includes amagnet 10 to impose the static magnetic field (B₀), gradient coils 12for imposing spatially distributed gradient magnetic fields (G_(x),G_(y), and G_(z)) having gradients along three respective orthogonalcoordinates, and RF coils 14 and 16 to transmit and receive RF signalsto and from selected nuclei of the body being imaged. The patient 18lies on a patient table 20 such that a portion of the patient to beimaged is moved, in three-dimensions, into an “imaging volume” betweenthe magnet and coils, which defines a field of view (FOV) of the MRIsystem. One or more sensors, such as electrocardiogram (EKG) sensor 21,may be positioned on the patient to monitor physiological conditions ofthe patient, such as the heartbeat.

[0004] The MRI system operator controls the system through a computerworkstation 22 with a keyboard, screen and other operator input/outputdevices. The MRI system operator positions the patient within theimaging volume using a movable table 20, and may attach sensors 21 thatmonitor the patient during imaging.

[0005] Sensors 21 monitor the heartbeat, respiration, blood pulse and/orother physiological conditions of the patient. Signals generated bythese sensors may be applied by the MRI system to trigger or synchronizeMR imaging with the physiological condition(s) being monitored. Forexample, a heartbeat sensor generates a signal indicative of thepatient's heartbeat that is applied to trigger an MR imaging sequencesynchronized with the beating heart. Synchronization of an MR image witha beating heart may improve the clarity of an still image of the beatingheart or enable a real-time image of the heart. Similarly, sensorsmonitoring physiological conditions may be used to synchronize MRimaging with respiration, blood pulses, and other conditions of thepatient. The signal from the monitor may also be recorded synchronouslywith the image data and used for post-processing.

[0006] Sensors to monitor physiological conditions are well known. Forexample, EKG electrical sensors mounted on a patient's skin detectelectrical signals from the heart and generate signals indicative of theheartbeat. Fluid flow sensors mounted near the nose or mouth of apatient detect a patient's breath and generate a respiration signal.Similarly, electromechanical sensors mounted on the chest or back of apatient detect changes in the shape of the abdomen to generate signalsindicative of the respiration of the patient. Blood pulse can bedetected by light sensors that detect light reflected from skin or bypressure sensors that detect pressure changes in an inflated bladderwrapped around the patient's arm.

[0007] The placement of such sensors on the patient's body can becritical. If the sensor is not placed optimally, then the triggering ofMRI scans can be affected. For example, the analog signal generated bythe sensor might be of insufficient magnitude to even pass through adetection threshold. Alternatively, the passage through a presetthreshold may be incorrectly timed and/or unreliable. Thus, the sensorplacement should be checked for correctness before expensive and timeconsuming actual MRI scans are conducted.

[0008] Providing an audio and/or visual signal feedback indicative ofproper sensor placement on a patient prior to MR imaging can bedifficult. MR imaging is extremely sensitive to stray electromagneticemissions. Such emissions may be emitted by signal wires and circuitsassociated with sensors. To reduce interference due to extraneousemissions, wires and circuits within the MRI imaging room and especiallynear the patient are minimized. In particular, the wires within the MRimaging room are preferably limited to only those wires needed for thegradient coils and the RF coils. Other signal wires and circuits aregenerally precluded from the MR imaging room.

[0009] Conventional CRT oscilloscopes that are often used to provide avisual feedback of patient sensor placement cannot be used in thepresence of the strong Bo magnetic field within an MRI gantry room. Inaddition, the additional wires and circuits associated with theoscilloscope may, if left in the gantry room, cause unnecessaryinterference with the MRI process—and thus must typically be located inan inconvenient place outside the gantry room. Similar problems ofinconvenience occur if an MRI system monitor outside the gantry room isutilized to monitor sensor placement. Moreover, even if the oscilloscopeor MRI system monitor were used, it would add to the cost of the MRIsystem. Still further, to view an auxiliary monitor, its visual displayrequires that an operator turn away from the MRI imaging screen todetermine whether the sensor is properly placed and functioning. Thus,while in-room monitors have been developed for MRI systems, (See U.S.Pat. No. 5,184,074, issued to Kaufman et al.) they have variousdisadvantages such as cost, complexity and possible stray RF emissions.

[0010] In MRI it is sometimes desirable to trigger or synchronize theimage acquisition sequence with a physiologically generated signal, suchas the heartbeat, the blood pulse or the respiration. The typical MRIsystem provides an input where a digital ON-OFF signal is used to createwithin the machine such triggering or synchronization.

[0011] To generate this digital signal, an analog signal is firstcreated by direct monitoring of the physiologic process of interest.Examples are EKG detection for the heartbeat, breath or abdomen shapefor breathing, and pressure or reflected light for blood flow. Thesignal these processes create is continuous, and a trigger level orthreshold has to be set to extract from them a desired ON-OFF signal.

[0012] The analog detection devices are attached to the body orotherwise interact with it, and because of patient-to-patientvariability, operators need to locate them in certain ways, or need totest different locations to get a reliable signal. Thus, the signalneeds to be monitored. This is typically done with an oscilloscope or bydigitizing the continuous signal and displaying it on the MRI consolemonitor.

[0013] Oscilloscopes themselves need adjustment that is sometimes beyondthe capabilities of the MRI operators, they add cost, and they do notwork near the MRI magnets. The console display is reliable and simple,but requires that the operator move between the patient and the console,which is in a different room. This adds time to the setup process. Anin-room monitor for real time display (U.S. Pat. No. 5,184,074) could beused to display the signal, but it adds cost.

[0014] There is a long-felt need for an economic device that providessensor signal feedback, such as of an EKG heartbeat signal, to an MRItechnician within the MRI gantry room. It is desirable that this devicenot add extra wires and circuits, especially within the MR imaging room.

SUMMARY OF THE INVENTION

[0015] This invention provides immediate real-time feedback to anoperator of an MRI apparatus as to the adequacy of the placement of aphysiologic sensor on a patient being prepared to undergo MRI.Basically, a special imaging sequence—one not actually used forproducing any image data and perhaps consisting only of a single pulseof a single gradient coil—is gated (i.e., triggered) by the occurrenceof a specific physiological phenomenon detected by a sensor placed onthe patient. Since gradient coils typically produce a characteristicsharp noise when they are pulsed, the triggering of the special imagingsequence results in the production of an immediately noticeable andreadily identifiable sound.

[0016] In practice, the physiological phenomenon-gated special imagingsequence mode is initiated just prior to placement of the sensor on thepatient. The operator is then free to test different placements of thesensor. If the sensor is improperly placed, the special imaging sequencewill not be triggered and the characteristic gradient coil noise willnot be produced.

[0017] When the sensor is properly placed, however, and the specialimaging sequence is properly triggered by the desired physiologicalprocess of the patient, the characteristic sound of the gradient coilnoise will be audible and will follow the rhythm of the patient'sphysiological process. In this manner, as the operator adjusts thesensor placement, the triggered imaging sequence sounds can be used asreal-time audible feedback to guide the operator as to the adequacy andreliability of the selected physiological process or placement of thesensor. In addition, the method of the present invention may be easilyimplemented on most types of MRI equipment for little or no additionalexpense.

[0018] The present invention provides an essentially zero cost feedbacksignal for determining optimism positioning of sensors in an MRI gantryroom. The invention does not add any wires or circuits to the MRIsystem, because it uses only an existing MRI gradient coil to generatean audible sound—triggered by the sensor as currently applied to apatient. Because it relies on an existing MRI system coil, the inventionis inexpensive (especially in view of conventional oscilloscope feedbackdevices). Moreover, the invention provides an audible sound that wasdirectly triggered by a heartbeat or other physiological condition. Thisimmediately and clearly notifies the MRI technician performing theplacement that the sensor is properly positioned and functioning—or, byits absence, that the sensor is not properly detecting the heartbeat orother physiological condition.

[0019] During this preparatory stage, the sensor signal is used by theMRI system to immediately trigger a special truncated test gated ortriggered sequence, e.g., single pulse, applied to one of the gradientcoils. This causes the gradient coil to vibrate, e.g., “ring”, andgenerate a distinct sound that can be readily heard by the MRI operatorin the gantry room. If the sensor is improperly placed or ismalfunctioning, then it will not trigger the special test sequence andthere will be no distinct sound from the gradient coil.

[0020] Obtaining good triggering signals from sensors applied to apatient can be problematic. To obtain a good signal, the sensors must beproperly positioned on the patient. Positioning sensors on the patientto detect physiological conditions generally requires that the MRIsystem operator be able to see or hear a characteristic trigger signalgenerated by the sensor as feedback to the sensor positioning process.By hearing or seeing such a trigger signal indication from the sensor,the operator can determine when the sensor is properly placed on thepatient. For example, EKG sensors must be positioned on the patient soas to properly detect electrical signals from the heartbeat. Todetermine whether the sensors are properly positioned, the operatorplaces the sensors on the patient's chest in the expected vicinity ofwhere the sensors can detect a heartbeat signal. The sensors may beconnected to an EKG monitor so that the operator can then see or hearthe heartbeat signal being detected by the sensors. If the signal is notclear and/or strong, the operator can move the sensors around on thechest until the detected EKG signal is strong and clear. Similarly,before a sensor-triggered MRI scan is conducted, the technician placingthe sensor(s) on the patient in the MRI gantry room needs to havefeedback to be sure that the sensor(s) is(are) properly placed. Withthis invention such feedback is provided by audible signals convenientlyand immediately in the gantry room and at essentially zero added cost.Thus, the technician does not need to go back and forth between thepatient and a monitor outside the gantry room.

[0021] The acoustic gated monitor is only used during patient setup. Theoperator has to be in the room for setting up the sensor system, e.g.,EKG, finger plethismograph, pressure cuff, breath flow, whatever. Thisrequires adjusting for picking up a physiologic signal. How does theoperator know if the placement is correct? Having an oscilloscope in theroom is inconvenient: if it is not shielded it has to be brought in forthe setting up and taken out before imaging starts, and the magneticfield affects it. Going out to look at the console monitor is veryinconvenient looking at a scope that is a distance from the magnet whileplaying with the patient sensor in or near the magnet is inconvenient.So the operator turns on one (or more) gradient sequence (without RF tocause interference) and listens as sensor adjustments are made. Theoperator positions the sensor on the patient. When the sensor iscorrectly positioned, the gated pulses to the coils generate soundsheard by the operator. These sounds inform the operator that the sensoris properly positioned. Once the proper positioning of the sensors isconfirmed by the sounds from the coil, the operator leaves the gantryroom and starts a conventional imaging sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The purpose and advantages gained by the present invention willbe understood by careful study of the following detailed description ofa presently preferred exemplary embodiment with particular reference tothe accompanying drawings, of which:

[0023]FIG. 1 is a schematic diagram of an exemplary MRI system employingthis invention, and

[0024]FIG. 2 is a gradient pulse timing diagram of an exemplary testgradient pulse sequence triggered by a sensor signal for the system ofFIG. 1.

DETAILED DESCRIPTION OF A PRESENTLY PREFERRED EXEMPLARY EMBODIMENT

[0025]FIG. 1 depicts an MRI system 1, such as the Toshiba OPART™ MRIsystem. An MRI system may comprise a large polarizing magnet structure10 which generates a substantially uniform homogeneous polarizing staticmagnetic field (B₀) surrounding a patient imaging volume 11. A suitablecarriage, e.g., patient table 20, inserts a portion of the anatomy ofthe patient 18 within the image volume 11. A sensor 21 may be applied tothe patient 18 so as to detect a physiological condition of the patient,such as heartbeat, respiration and/or blood pulses flowing through thecirculatory system. The patient, patient table (bed) 20, magnet 10, andcoils 12, 14, 16 are housed in a shielded gantry room 40 that isisolated from external RF and electromagnetic interference. From thisroom, a technician prepares the patient for imaging, positions thepatient on the bed and attaches sensors to the patient. The techniciangenerally does not have monitoring electronics, such as a computerdisplay screen, available in the gantry room. Monitoring electronics cangenerate RF signals that would interfere with MR imaging and/or suchmonitoring equipment (e.g., a CRT) may be inoperative in the presence ofthe strong polarizing magnetic field B₀. Thus, proper positioning of thesensor 21, without this invention, can become an awkward, tedious taskrequiring iterative sensor positioning and walking to/from the monitoroutside the gantry room. This invention eliminates such problems. Oncethe patient is prepared for imaging, the technician/operator exits thegantry room 40 and controls the imaging sequence from a workstation 22.

[0026] Gradients in B₀are selectively created by electromagneticgradient coils 12 that are operated by an MRI sequencing processor 26.RF nuclei nutation pulses are transmitted into patient tissue within theimage volume by RF coil 14. RF responses constituting the MR signal arereceived from the patient tissue via suitable RF detection coils 16.

[0027] The MRI system operator controls the system 1 through a computerworkstation 22 with a keyboard, screen and other operator input/outputdevices. The workstation is generally located outside of the shieldedimaging room 40. The MRI workstation 22 is electronically connected toan MRI system computer 24 which controls the MRI system. The computerconverts operator MRI image sequence parameter selections into suitablecommands for operation of the MRI system. In particular, the computercontrols the selection of an appropriate gradient pulse program module26 (e.g., from an associated memory device) which, in turn, applies aselected magnetic pulse sequence(s) to the gradient amplifiers 28 thatdrive the gradient coils. The MRI pulse sequences may be triggered by asensor output signal.

[0028] In this invention, a special truncated test sequence may beselected for use during patient setup that causes the computer 24 tocommand the pulse program module 26 to apply a non-imaging pulsesequence (e.g., of one immediate pulse) to one of the gradient coils 12for the sole purpose of causing the coil to vibrate and emit a soundthat coincides with the currently available trigger sensor output, ifany.

[0029] Since the generation of a truncated (e.g., single) pulse sequenceis considerably more simple than that of a traditional MRI pulsesequence, it is not necessary to further describe the exemplary simplesoftware for implementing such a test mode of operation. Those in theart will have no difficulty devising a suitable truncated test sequencein a suitable portion of the MRI system memory.

[0030] In an MRI system, various coils produce RF excitation pulses andaccompanying gradient field pulses result in and acquire an MR signalduring an MRI “acquisition sequence” using well-known MRI techniques.Each acquisition sequence (or series of sequences) may be triggered byan event, such as a heartbeat signal from an EKG sensor 21 (FIG. 1).

[0031] The physiological sensor 21 applied to the patient monitors acondition, such as the patient's heartbeat, respiration, and/or bloodpulse in the circulatory system. As the sensor detects the occurrence ofthe heartbeat, respiration or blood pulse, if properly placed, itgenerates a sensor trigger signal 62 to the MRI computer 22. Thecomputer 22 may apply the sensor signal to trigger an MRI acquisitionsequence or to inhibit an acquisition sequence. For example, an EKGsignal may be used to trigger a sequence where the sequence is tocorrespond to the heartbeat. In another example, a respiration signalmay be used to inhibit an acquisition sequence where a patient'smovement during respiration might blur an image.

[0032] However, in the invention, a special sensor test mode is providedwherein a gradient pulse 64 is applied to acoustically “ring” one ormore of the gradient coils. This test ring” pulse is triggered (gated)by a sensor signal 62 that is generated, e.g., by a heartbeat sensor,positioned on the patient. The audible sound caused by gradient pulse 64then immediately notifies the MRI system technician/operator in thegantry room (or elsewhere) that the heartbeat sensor is functioningproperly and is detecting a heartbeat signal. If the operator does notperiodically, e.g., once every {fraction (1/60)}^(th) of a second intime with a manually-detected patient pulse, hear an audible ring fromthe gradient coil, then the operator can assume that the heartbeat isnot being properly detected by the sensor. The operator can then decidewhether the sensor should be repositioned or otherwise serviced.

[0033] The use to which the sensor signal is applied will depend on thetype of signal, e.g., EKG heartbeat signal, respiration and blood pulse,and the mode of MRI operation selected by the operator. Moreover, in theinitial test mode, only a non-imaging sequence (intended to onlygenerate an audible sound) need be triggered by the sensor. Inparticular, upon receipt of a sensor trigger signal 62, the computer 22may cause the gradient pulse program module 26 to generate a specialtest pulse sequence 64 that causes one of the gradient coils to vibrate(“ring”) and thereby create an audible sound. This sound may be heard bythe MRI system operator as an indication that the sensor is properlydetecting the desired physiological condition of the patient.

[0034] The test sequence may have no purpose other than to audibly ringthe gradient coil, and may not influence the regular MRI pulsesequences. Indeed, the gradient pulse 64 may be applied without anyradio frequency (RF) pulses. Generally, there will be no imaging whilethe operator is prepping the patient on bed 20 in the gantry room. Thus,there would typically be no need or desire to apply full MRI pulsesequences (e.g., including RF pulses) to the coils of the MRI system.

[0035] The noise caused by vibration of the gradient coil is a common,normally adverse, side effect of applying pulse sequences to gradientcoils. However, the present invention make constructive use of such coilsounds by providing those sounds as audible feedback from a sensor 21trigger output.

[0036] After a patient has been prepared for imaging and sensors havebeen properly placed on the patient (using the coil sounds generated bythe present invention), the operator leaves the MRI gantry room andenters the MRI control room. From the control room, the operatorinitiates and controls MR imaging as usual and conventional.

[0037] While the invention has been described in connection with what ispresently a preferred exemplary embodiment, it is to be understood thatthe invention is not to be limited to the disclosed exemplaryembodiment, but on the contrary, is intended to cover all modificationsand variations of the invention apparent to those skilled in the artfrom this disclosure, including those coming within the scope of thefollowing appended claims.

What is claimed is:
 1. A method for magnetic resonance imaging (MRI)using an MRI system having at least one coil, said method comprising: a.positioning a patient within the MRI system for MR imaging; b. couplinga sensor to the patient and provide a sensor output signal representinga physiological event in the patient; and c. applying at least oneelectrical pulse to the at least one coil triggered by said outputsignal of the sensor, and thereby emitting an audible sound from the atleast one coil.
 2. A method as in claim 1 further comprising adjustingthe sensor on the patient if the sensor does not reliably trigger saidaudible sound.
 3. A method as in claim 1 wherein the sensor detectspatient heartbeat, and the audible sound coincides with the heartbeat.4. A method as in claim 1 wherein the coil is a magnetic gradient fieldcoil.
 5. A method as in claim 1 wherein the sensor detects patientrespiration, and the audible sound coincides with the patient'srespiration.
 6. A method as in claim 1 wherein the sensor detects ablood flow pulse in the patient, and the audible sound coincides withthe patient's blood flow pulse.
 7. An magnetic resonance imaging (MRI)system comprising: an MRI imaging volume bounded by a static magneticfield (B₀) and adapted to image a patient at least partially within thevolume; at least one magnetic gradient coil; a sensor applied to detecta physiological event in the patient; a controller having a processorand an associated memory, said controller being operatively coupled tosaid at least one gradient coil and said sensor, and said memory storingdata representative of at least one truncated test pulse sequence to beapplied to said at least one gradient coil, wherein said controllergates the at least one truncated test pulse sequence to the gradientcoil upon receipt of an output signal from said sensor.
 8. An MRI systemas in claim 7 wherein the sensor is a heartbeat sensor.
 9. An MRI systemas in claim 7 wherein the sensor is a respiration sensor.
 10. An MRIsystem as in claim 7 wherein the sensor is a blood pulse sensor.
 11. Amethod for placing physiological event sensors on a patient beingprepared for MRI scanning, said method comprising: a) placing one ormore physiological event sensors on a patient; b) using output of thesensors, if present, to trigger an audible signal from a coil of an MRIsystem normally used for MRI scanning; and c) repeating a) and b) asnecessary to achieve proper placement of said one or more sensors on thepatient prior to performing an MRI scan of the patient.
 12. An MRIsystem having a preparatory test mode for assisting a technician inplacement of physiological event sensors on a patient being prepared forscanning, said system comprising: a programmed controller coupled to RFcoils and magnetic gradient coils via associated circuits for acquiringMRI data during an MRI scan of the patient; and program memory coupledto said controller including a test truncated pulse sequence which, whenactive, is triggered by at least one of said physiological event sensorsto activate at least one of the gradient coils and thus produce anaudible sound associated with a detected physiological event, if any.